1. Field of the Invention
The present invention relates to technology for shimming during magnetic resonance imaging (MRI) and, particularly, to a method and an apparatus for compensating an insufficient homogeneity of an MRI basic magnetic field using shimming technology.
2. Description of the Prior Art
For a medical MRI system, it is necessary to place a patient in a homogeneous magnetic field, so as to orient the proton magnetic moments of water molecules in the body along a single direction, then the imaging region is irradiated with radio frequency signals that are absorbable by the protons. A greyscale image of the tissue within the region is acquired by applying a radio frequency sequence and a gradient sequence, receiving the resulting resonance signals, and carrying out signal processing. According to the conditions of magnetic resonance, the basic magnetic field of the imaging magnet needs to exhibit a relatively high homogeneity. The more homogeneous the basic magnetic field, the higher the signal-to-noise ratio (SNR).
With the increasing enhancement of MRI field intensity, the requirement on the SNR also increases, therefore shimming technology becomes more and more important. During practical production, the magnetic field of a magnet cannot reach the uniformity level required for MRI imaging, and for this reason, it is necessary to carry out shimming after the magnet has been assembled. An MRI magnet for common medical whole-body imaging needs to form a uniform net magnetic field within a DSV (diameter spherical volume) of 30-40 centimeters from the center of the magnetic field.
Although a homogenized magnetic field will be formed within the spherical volume at the center of the magnetic field and the homogenized magnetic field can cover most parts of a body, it is still possible for some body parts, such as the shoulders, to be exposed outside the homogeneous magnetic field, so the imaging quality of such body parts exposed outside the homogenized magnetic field will be significantly affected.
To address this problem, solutions currently available include mainly the following:
(1) Design a magnet that is capable of forming quite a large homogenized magnetic field, for example, a homogenized magnetic field up to a spherical volume of 50 centimeters in diameter, so as to cover the body parts such as shoulders; however, this method is very expensive.
(2) Make a rational arrangement of the patient inside the magnet, so as to position the body parts concerned for diagnosis, such as shoulders, within the region of the homogenized magnetic field. The condition upon which this method is based is that there is enough space in the magnet gap, making it possible to form the images of the patient's body parts, such as shoulders.
(3) Adopt a local shimming method so as to obtain a locally homogenized magnetic field at the body parts, such as shoulders, and such a technology is disclosed in the DE29805903, corresponding to U.S. Pat. No. 6,291,998. This method has some drawbacks: firstly, the locally arranged homogeneous field will produce magnetic force; secondly, the locally arranged homogeneous field needs a certain space and location accuracy; and thirdly, shimming by metal will produce eddy currents, thus affecting imaging quality.